Project summary- Activation of the PI3K signaling pathway is a common event in human breast cancer and is most commonly due to PI3K alpha mutation, HER2 amplification, or PTEN inactivation. Activation of the pathway plays an important role in oncogenesis and tumors with these lesions are dependent on pathway function, whereas tumors in which the pathway is not deregulated are not. While inhibitors of the PI3K pathway effectively suppress growth in vivo, they tend not to induce regression. Our proposal is based on the idea that physiologic adaptation to PI3K pathway inhibitors attenuates the antitumor effects of these drugs and that inhibition of the adaptation will markedly enhance their therapeutic effects. We have shown that constitutive activation of mitogenic signaling by oncoproteins is accompanied by exaggerated feedback inhibition throughout the signaling network. These high levels of feedback play important roles in the biology of transformation and in the response of tumor cells to targeted therapies. High signaling output causes profound inhibition of normal signaling pathways and this causes the cell to be hyperdependent on the oncoprotein dependent pathway. This is responsible in part for the initial sensitivity of these tumors to pharmacologic inhibition of this pathway. But while the tumor is initially sensitive to inhibitors of the activated pathway, inhibition relieves feedback, reactivates upstream signaling and this, we believe, attenuates the antitumor effects of these drugs. Our previous data shows that inhibitors of different nodes in the PI3K pathway (PI3K, AKT, mTOR) all relieve feedback and that combined inhibition of PI3K signaling and adaptive receptor reactivation has enhanced therapeutic activity and causes tumor regression. We now propose in Aim 1 to characterize the relief of feedback responses to selective inhibition of PI3K, AKT or mTOR. In Aim 2 we will use genetic and pharmacologic modalities to determine which adaptations are most responsible for maintaining the survival of the tumor in which the oncogenic pathway has been inhibited. In Aim 3, the data will be used to identify combination therapies based on this concept and test and optimize their antitumor effects in vivo